Loss of stem cell self-renewal results in failure to maintain organs and can lead to degenerative disorders. In contrast, loss of differentiation can lead to cancers. Thus, the ability to prevent premature differentiation in degenerative diseases, or to induce differentiation in case of cancer, will have tremendous therapeutic impact. Our long-term goal is to determine key regulatory pathways that control the transition from stem cell self-renewal to differentiation, using Drosophila germline stem cells (GSCs) as a model system. The germ cells are the ultimate stem cells as they are both totipotent as well as immortal. Thus, paradigms established in the germ line can be extended to other stem cell systems. Drosophila is a superior model system to study questions about stem cell self-renewal and differentiation because of the availability of mutants, markers, RNAi technology and targeted expression methods. In Drosophila embryogenesis, the conserved process of global transcriptional silencing, mediated by the gene polar granule component (pgc), plays a pivotal role in germ cell specification. In absence of pgc, germ cells show precocious transcription that result in the transcription of somatic genes leading to their death. During oogenesis, an oocyte fate is being specified from a GSC fate during the process of differentiation. We have discovered that during oogenesis, Pgc is transiently expressed in the differentiating daughter of the GSC and loss of pgc results in differentiation defects. We propose that during oogenesis, Pgc mediated transcriptional silencing acts to suppress the response to self-renewal signaling from the surrounding niche cells and promotes differentiation in the GSC daughter. To test this hypothesis we will (1) Determine how the transcriptional silencer, Pgc, promotes GSC differentiation; (2) Identify Pgc targets and the mechanism of action during GSC differentiation; (3) Investigate how pgc is regulated during oogenesis. Our work will establish a role for transient transcriptional silencing in reprogramming stem cell fate by demonstrating that Pgc in the GSC daughter is expressed in a cell cycle dependent manner and determining a requirement for Pgc to promote efficient differentiation. We favor the idea that transcriptional silencing is needed to clear residual stem cell factors, thereby reprogramming the GSC daughter prior to differentiation. Based on findings from this application, we posit that transcriptional silencing could be a potent target for increasing efficiency in deriving stem cells and also as a target in cancer treatment.